1. Field of the Invention
The present invention relates generally to a light emitting diode (LED) lighting device. The present invention further relates to an LED assembly having a point-source of light to simulate an incandescent lamp.
2. Description of the Related Art
Reliable safety lights are critical for the safety of boats to prevent accidental collisions during darkness and inclement weather. The vast majority of marine safety lights, such as the one disclosed in U.S. Pat. No. 5,711,591 issued to Jordan use incandescent light bulbs as the light source.
A number of attempts have been made to replace marine filament bulbs with LEDs in marine safety lights because of their relatively small power consumption and long life. Incandescent lamps radiate all of their light on a horizontal plane from the filament in the center of the lamp. This type of source is called a point-source.
Incandescent bulbs have a resistant tungsten filament suspended by support wires with a vacuum inside a glass bulb. As a result, they are highly susceptible to damage due to temperature variations and vibrations. The typical life of incandescent bulbs usually averages one or two thousand hours, so that they must be replaced several times a year.
LEDs, on the other hand, are more efficient than incandescent bulbs at converting electricity into light. LEDs are also more durable and immune to filament breakage due to shock or vibration. Therefore, LEDs have a life span of approximately 50,000 hours versus one to two thousand hours for an incandescent bulb. This means that the bulbs do not have to be replaced nearly so often and do not require much maintenance. This is particularly important for marine lanterns that are difficult to access.
LEDs, however, are not without their problems. Several of these problems are discussed in a paper entitled Design Considerations for Reliability and Optical Performance of LED Signal Lights given by Paul F. Mueller at the XVth IALA Conference, March 2002.
A first problem is that typical, low-output, 5 millimeter LEDs (currently available in lighting devices such as those used for marine and airport safety lights) only have a driving current ranging from about 50 to 70 milliwatts and produce insufficient lumens or candlepower to meet the 3-4 mile visibility requirement. Although it is possible to increase the optical output considerably by increasing the forward current above the nominal rated value, such an increase in forward current generally leads to premature failure of the LED due to overheating of the diode junction. Recently, however, high-output LEDs (driving current of about 1-5 Watt with a high lumens output) have become available.
A second problem is that LEDs have a poorly directed, non-uniform and excessively divergent pencil beam pattern. It is customary to produce a 360° beam pattern of superimposed pencil beams by arraying multiple LEDs in a circular, outward-directed pattern. While this provides an omni-directional beam pattern, lacking further optical enhancement, the result is energy inefficient and grossly non-uniform in horizon intensity.
There are several major manufacturers that produce marine lanterns with LEDs including: Carmanah Technologies, Inc., Zeni Buoy Light Company Limited, Vega Industries Limited, Tideland Signal Corporation, and Sabik Oy. All of the currently available marine lanterns using LEDs use low output LEDs. Thus, all of these lanterns require large numbers, up to several hundred, of LEDs to produce the minimal total flux (lumens or candlepower) necessary for a marine lantern.
Marine LED lanterns use multiple arrays of multiple LEDs that do not have a single point source of light and cannot use a fresnel lens to capture and focus the light from the LED arrays used. All five of the manufacturers mentioned above have been required to design new lenses to capture and focus the light from their LED arrays. One approach to this problem has been to design a fine lens incorporated in front of the LEDs to converge the beam of light and increase the luminance thereof. For example, U.S. Pat. No. 5,224,773 discloses a thin fresnel lens made by rolling and welding the edges of a thin, transparent film of acrylic resin with a fine-pitched surface that is formed by heating and pressing a mold for a thin linear fresnel lens to form a cylinder.
Alternatively, U.S. Pat. No. 6,048,083 issued to McDermott describes an optic lens that is contoured to create a plurality of focal points which form a bent or crooked focal line cooperate with the orientation of the LED elements to project a composite light beam with limited divergence about a first reference plane.
Another approach has been to construct a small marine safety light that has a much lower candlepower. U.S. Pat. No. 6,086,220 issued to Lash et al. describes a marine safety light having six or more low output LEDs having a uniform star configuration. The inventors determined that such an LED array produced visible light over one nautical mile away from the vessel, whereas most marine lanterns must meet a 60 candela requirement for a three to four mile visibility.
There is an existing need for a lighting device that replaces the incandescent bulb with LEDs that have sufficient candlepower and that provides an omni-directional beam pattern. A point-source LED light-source assembly is desirable for retro-fitting existing lanterns and lenses designed for point-source lighting.